Method for producing sodium borates of lowered iron content



United States Patent @fifice 3,067,113 Patented Dec. 4., 1962 3,667,113lviE'ii-IQD PRQBUCKNG S IBDEUM @F LGWEPED IRON CGNTENT Ray B. Fenland,Spokane, Wash, assignor to Unite States Borax & @Iihemical Corporation,Los Angeles, (lalit, a corporation of Nevada No Drawing. Filed May 1,1961, Ser. No. 106,484 2 Claims. (Cl. 204112) The present inventionrelates to a method for reducing the iron content of sodium boratesolutions.

Most commercial sodium borates, as for example, sodium tetraboratedecahydrate and sodium tetraborate pentahydrate are produced from orescontaining the crude borates combined with a gangue comprising clay andsmall amounts of various other impurities.

The common method for the recovery of the sodium borates is a wetprocess in which the ore is dissolved in water or mother liquor, treatedwith carbonate ion and separated from the gangue by subsequentscreening, settling and filtration. The finished sodium borate productsare then usually obtained from the clarified solution bycrystallization. The clarified sodium borate solutions contain on theorder of about 20-50 parts per million total iron and the finishedproducts when crystallized from the solutions contain on the order offrom about -30 parts per million total iron. For most of the commercialuses of the sodium borates this small quantity of iron is of noimportance; however, there are various requirements for sodium borateswherein it is necessary that the iron content be on the order of 8 partsper million and less.

it is, therefore, the principal object of the present invention toprovide a wet process method for producing sodium borates having alowered iron content.

Other objects of the present invention will appear as the descriptionproceeds.

To the accomplishment of the foregoing and related ends, said inventionthen comprises the features hereinafter fully described and particularlypointed out in the claims, the following description setting forth indetail certain illustrative embodiments of the invention, these beingindicative, however, of but a few of the various ways in which theprinciple of the invention may be employed.

Broadly stated, the present invention comprises the method of producingsodium borates having a lowered iron content which comprises placing anaqueous solution of sodium borate, containing trace amounts of solublesulfur compounds and additionally containing iron as a contaminant intoan electrolytic cell, passing an electric current through the solutionat an applied potential that exceeds the decomposition potential of ironin said solution, removing said iron as a combination of metallic ironand iron sulfide at the cathode, separating said sodium borate solutionfrom any metaliic iron and iron sulfide flakes and crystallizing sodiumborate from the purified solution.

From the foregoing broadly stated paragraph it can be seen that thepresent method for removing iron contaminants from aqueous sodium boratesolutions is predicated on the electrolytic deposition of iron fromaqueous solutions. Although it is Well known that iron can be removedfrom aqueous solutions by electrolysis, such a method is normally quiteslow and usually requires considerable amounts of electrical power whenthe concentration of iron. is low, in the range of from to 100 parts permillion, and it becomes even more difficult to perform as theconcentration of the iron present diminishes.

The present invention, however, prov-ices a method for the electrolyticremoval of iron from aqueous sodium bot-ate solutions in such a manneras to increase the iron deposition rate by a factor of from about 5 to12, and thereby lowering the power requirement substantially whileproducing a more highly purified sodium borate solution. Although theexact reaction mechanism for this process is not clearly understood, Ihave found that this can be accomplished by performing the electrolysisin the presence of sulfur compounds, wherein the sulfur is in some formother than sulfate.

When the electrolysis of the contaminated sodium borate solutions isperformed in the presence of sulfur compounds the deposits on thecathode are found to be in the form of a combination of metallic ironand iron suifide. These deposits occur even when the sulfur is notpresent as sulfide per se, but in the form of a com plea or polysultide.The formation of insoluble iron sulfide is quite surprising since thishas been found to be a very slow and most difiicult reaction to performin sodium borate solutions having low iron concentrations.

Since the total iron to be removed from the solution is on the order offrom about 2% to 50 parts per million the amount of sulfur required inthe present process is small, and since the iron is removed as acombination of metallic iron and iron sulfide, the amount of sulfurrequired is less than the equivalent amount required to react with thetotal iron present. Some of the sodium borate ores contain enoughsoluble sulfur compounds as impurities, so that they themselves supplyan adequate quantity of sulfur to the solution for performing thecataiyzed removal of iron by electrolysis. However, when electrolysiug asodium borate solution which is free of sulfur, the sulfur is added inthe form of a watersolu'ole salt other than a sulfate.

The electrolytic cell used in the present process can be of the mostsimple design and construction, identical to the well known electrolyticcells such as those used in the electrolytic preparation of hydrogen,and oxygen. Since the sodium borate solutions are non-corrosive to ironan inexpensive cell would be comprised of an iron or mild steel tank andsince iron is removed at the cathode, the cathode can also be made fromiron or mild steel. Many materials were tested as anodes and suchmaterials as graphite, lead, nickel, tungsten, and platinum as well asother materials were found applicable to the present invention.

The prose 3 method for preparing low iron sodium borate soiutions byelectrolysis is suitable for both batch type and continuous flow typeoperations, and in the preferred embodiment of the invention i use acontinuous flow system. in a batch type operation, the solution, freefrom the 'gangue and containing a minimum of insoluble mat rial, isplaced in the electrolytic cell and held there until the desired ironconcentration, less than about 10 parts per million, is reached. In acontinuous flow operation, the substantially solids free solution iscontinuously passed through an electrolytic cell of such capacity as toprovide a proper residence time for the desired electropurification.

In either type of operation the iron is found to be deposited as acombination of metallic iron and iron sulfide on the cathode of theelectrolytic cell. However, in both systems a certain amount of thedeposits flake off the oathode due to excessive build up and attrition.In the case of the batch type operation the deposits settle to thebottom of the cell and are separated from the solution by decantation,while in a continuous system much of these solids are carried into theflow stream from which they are readily removed by filtration. Thepurified and clarified solutions containing less than about 10 parts permillion iron, are then ready to undergo crystallization, from which asatisfactory product containing less than about 8 parts per million ironcan be obtained.

So that the present invention is more clearly understood, the followingexamples are given for illustrative purposes.

The electrolytic cell used in the following examples comprised an ironbox having an inlet at the bottom of one end of the box and an overflowoutlet at the other end. The box was constructed so that screen typeelec trodes could be fitted into the box, and when in operation theanodes and cathodes were placed alternately in the cell. The electrodeswere connected to a variable power source and the flow rate of thesolution was controlled by a variable pump. Each of the examples wasconducted continuously for 5 to 6 hours with samples of the efiiuentbeing taken every 15 minutes to make up a large composite sample foranalysis.

A 43% sodium tetraborate decahydrate solution containing 19.4 parts permillion iron and about 15 parts per million sulfide was prepared. Thesolution, containmg about 0.015% solids, was pumped through theelectrolytic cell at the rate of 1.98 gallons per hour, equivalent toabout a 6.5 minute residence time. The power supplied to the electrodeswas held constant at 3.0 volts and 4.0 amperes for the entire run. Theeffiuent was filtered and the composite sample was found to contain 8.2parts per million iron. Thus after 5 hours at a power requirement of 12watts, 9.9 gallons of solution containing 8.2 parts iron was obtained. H

A 42.6% sodium tetraborate decahydrate solution containing 34.4 partsper million iron and about 20 parts per million sulfide was prepared.The solution, containing about 0.013% solids, was pumped through theelectrolytic cell at the rate of 1 gallon per hour, equivalent to abouta 13 minute residence time. The power supplied to the electrodes washeld constant at 4.2 volts and 3.2 amperes for the entire run. Theefilucnt was filtered and the com posite sample was found to contain 5.6parts per million iron. Thus after 5 hours at a power requirement of13.4 watts, 5 gallons of solution containing 5.6 parts iron wasobtained.

III

A 43.8% sodium tetraborate decahydrate solution containing 34.3 partsper million iron and about 30 parts per million sulfide was prepared.The solution, containing about 0.018% solids, was pumped through theelectrolytic cell at the rate of 1.82 gallons per hour, equivalent toabout a 7 minute residence time. The power supplied to the electrodeswas held constant at 3.6 volts and 2.5 amperes for the entire run. Theeffluent was filtered and the composite sample was found to contain 8.0parts per million iron. Thus after 5 hours at a power requirement of 9watts, 9.1 gallons of solution containing 8.0 parts iron was obtained.

A 44.1% sodium tetraborate decahydrate solution containing 28.0 partsper million iron and about 18 parts per million sulfide was prepared.The solution, containing about 0.010% solids, was pumped through theelectrolytic cell at the rate of 2.54 gallons per hour, equivalent toabout a 5 minute residence time. The power supplied to the electrodeswas held constant at 3.5 volts and 2.5 amperes for the entire run. Theefiluent was filtered and the composite sample was found to contain 10.3parts per million iron. Thus after 5 hours at a power require Thisexample was done to compare the previous examples with the electrolysisof a sulfur free sodium borate solution. A 43% sodium tetraboratedecahydrate solution containing 29.5 parts per million iron and about0.015% solids was prepared. The solution was passed through theelectrolytic cell at the rate of 0.46 gallon per hour, equivalent toabout a 28 minute residence time. The power supplied to the electrodeswas held constant at 3.2 volts and 6.0 amperes for the entire run. Theeifiuent was filtered and the composite sample was found to contain 17.9parts per million iron. Thus, with the sodium borate solution containingno sulfur, after 5 hours at a power requirement of 19.2 watts, only 2.3gallons of solution containing 17.9 parts per million iron was obtained.

it will be seen from the foregoing examples that the electrolyticremoval of iron from aqueous sodium borate solutions is greatly enhancedby the presence of trace quantities of sulfur compounds. It will also benoted that the present method provides a rapid and economical processfor removing iron from such solutions which is readily adaptable to thecommonly used wet process for preparing sodium borates.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

I, therefore, particularly point out and distinctly claim as myinvention:

1. The method of producing sodium borates having a lowered iron contentwhich comprises placing an aqueous solution of sodium borate containingtrace amounts of soluble sulfur compounds and additionally containingtrace amounts of iron as a contaminant into an electrolytic cell,passing an electric current through the solution at an applied potentialthat exceeds the decomposition potential of iron in said solution,removing said iron as a combination of metallic iron and iron sulfide atthe oathode, separating said sodium borate solution from any metalliciron and iron sulfide flakes and crystallizing sodium borate from thepurified solution.

2. The continuous method for producing sodium borates having a lowerediron content which comprises continuously passing an aqueous sodiumborate solution containing trace amounts of soluble sulfur compounds andadditionally containing trace amounts of iron as a contaminant throughan electrolytic cell while passing an electric current through thesolution at an applied potential that exceeds the decompositionpotential of iron in said solution, removing said iron as a combinationof metallic iron and iron sulfide at the cathode, separating saidsolution from any flakes of metallic iron and iron sulfide by filtrationand crystallizing sodium borate from the purified solution.

References Qited in the file of this patent UNITED STATES PATENTS1,793,906 Christensen Feb. 24, 1931 2,722,480 Kumar NOV. 1, 19552,776,184 Kamen Jan. 1, 1957 2,834,727 Gullet May 13, 1958 2,961,294Taylor ct al. Nov. 22, 1960 FOREIGN PATENTS 113,508 Great Britain Feb.28, 1918

1.THE METHOD OF PRODUCING SODIUM BORATES HAVING A LOWERED IRON CONTENTWHICH COMPRISES PLACING AN AQUEOUS SOLUTION OF SODIUM BORATE CONTAININGTRACE AMOUNTS OF SOLUBLE SULFUR COMPOUNDS A ADDITIONALY CONTAINING TRACEAMOUNTS OF IRON AS A CONTAMINANT INTO AN ELECTROLYTIC CELL, PASSING ANELECTRIC CURRENT THROUGH THE SOLUTION AT AN APPLIED POTENTIAL THATEXCEDS THE DECOMPOSITION POTENTIAL OF IRON IN SAID SOLUTION, REMOVINGSAID IRON AS A COMBINATION OF METALLIC IRON AND IRON SULFIDE AT THECATHODE, SEPARATING SAID SODIUM BORATE SOLUTION FROM ANY METALLIC IRONAND IRON SULFIDE FLAKES AND CRYSTALLIZING SODIUM BORATE FROM THEPURIFIED SOLUTION.